This invention relates to oscillating inertial microbalance mass measurement devices and, more specifically, to mass measurement devices that reduce the adverse effects that temperature, pressure, force, and modulus of elasticity have on the true indication of mass. It is known in the art that vibrating systems have a large dependency on their environment, materials of construction, and method of manufacturing.
A significant contribution to the adverse effects can come from a non-ideal oscillating element that is at the heart of the classical oscillating inertial microbalance. Exemplary construction and operation of such oscillating inertial microbalance mass measurement devices are disclosed in U.S. Pat. Nos. 4,391,338, 6,444,927, and 6,784,381 which patents hereby are incorporated herein by reference in their entirety.
An ideal oscillating element is one that has the ability to indicate the mass of the collected matter without the need to compensate for external outside acting forces such as temperature, pressure, modulus of elasticity, external vibration or any other force that may create an adverse effect on the true indication of the mass.
For instance, a non-ideal oscillating element can exhibit a change in the modulus of elasticity with temperature. It is difficult to precisely determine how to apply corrections for temperature when the adverse effects are comprised of those from the fluid properties of the sampled gas and those errors contributed by a change in the modulus of elasticity of the oscillating element has with temperature.
The problems associated with maintaining the gas stream and the oscillating element at a constant temperature are discussed in U.S. Pat. No. 6,080,939 and methods for controlling the same are discussed in U.S. Pat. No. 6,444,927
Further, instruments used to measure a particular parameter may be affected by the variation of other parameters. For example, measurement of the mass of material deposited on an oscillating inertial microbalance may be adversely affected by a variation in temperature and/or pressure and/or changes in modulus of elasticity of the oscillating element of the oscillating inertial microbalance.
A microbalance, examples of which are described in U.S. Pat. Nos. 3,926,271 and 4,391,338, typically comprises an oscillating element mounted with one end fixed and the other end free. The free end typically has a filter (or other mass-receiving element) mounted thereto.
When a microbalance is configured with a hollow oscillating element, the fluid is typically drawn through the filter and through the oscillating element, thereby trapping suspended particles within the fluid in or on the filter.
The resulting increase in the mass of the filter results in a decrease of the resonant frequency of the oscillating element. The decrease in the resonant frequency of the oscillating element is related to the increase in mass of the filter, which in turn is representative of the mass of the suspended particles trapped in or on the filter.
Because the oscillating element has the ability to continually indicate the mass of the suspended particles it is an ideal means for indicating the change in mass of the suspended particles trapped in or on the filter in near real time or over a measured period of time.
A microbalance, an example of which is described in U.S. Pat. No. 6,205,842, attempts to address the problem of the adverse mass indication as a result of the volatile components in the sample stream. The adverse mass indication is compensated for by having two substantially similar mass detectors where a particulate removal means is provided on one of the mass sensors to which the other is compared. A switching means is provided and equations are utilized to remove said adverse effects, enabled by the use of the switching means.
An attempt to remove the mechanical complexity of the switching means mentioned above is noted in U.S. Pat. No. 6,502,450, wherein only one sensor is used with the same particle removing means in a single flow path. The switching of a particle removal means and associated timing gives rise to the apparent loss in mass by using a formula. A further improvement of this patent, or a simplification to remove the adverse effects of volitization of the gas, is described in U.S. Pat. No. 6,651,480.
The goal of the three aforementioned patents is to address the adverse effects created by gas volitization in an inertial microbalance. They are not designed to remove the sensitivities an inertial microbalance has to the modulus of elasticity with temperature and the density change within the hollow oscillating element as a result of temperature and pressure. None of these patents provide for a continuous indication of the mass in near real time. Instead, they require that the particulate laden sample stream be interrupted, results in the disruption of the sampling cycle which can in turn potentially lead to the missing of an episode wherein a large concentration of the particulate sample is ignored.
Additionally, as the temperature of the microbalance's oscillating element changes, the resonant frequency of the oscillating element may change, even though the mass in or on the filter secured to the oscillating element may remain unchanged. As the measured mass is based on the resonant frequency, an error is introduced in the mass determination. This temperature sensitivity results primarily from a change in the modulus of elasticity of the materials from which the oscillating element is made. One way of addressing the concern of the temperature sensitivity of the microbalance, or other instrument, is to select a material of construction that has minimal sensitivity to changes in temperature.
For a microbalance, great care can be applied to the formulation of the material from which the oscillating element is made to attain the desired characteristics while attempting to optimize manufacturability and minimize the temperature sensitivity of the desired variables.
In particular, one way of reducing the temperature sensitivity of the microbalance known in the prior art is to use a shaped oscillating element made of a material having a low temperature coefficient of elastic modulus. In the end, compromises must be made at the expense of the accuracy, manufacturability and cost of the entire system.
U.S. Pat. No. 4,836,314 describes the selection of a material for an oscillating element fabricated from a glass of a specific composition. Additionally, U.S. Pat. No. 6,080,939 describes a process of heat treatment and material combination of a metallic material. Both of these patents fail to completely address the adverse effects of temperature sensitivity having to do with the modulus of elasticity of the oscillating element.
A method of compensation of adverse effects is described in U.S. Pat. No. 5,604,335 wherein the mass is only measured when the system is in a quiescent state wherein no flow is delivered to the oscillator. The dual sensor system fails to subject both oscillators to substantially the same conditions. Our invention specifically prescribes that both oscillators be subjected to substantially the same conditions.
U.S. Pat. No. 4,836,314 teaches a method of selection of the material of construction for reducing the thermal coefficient of elastic modulus over a selected temperature range. The patent shows a “recipe” for a glass alloy to accomplish a fairly low thermal coefficient of elastic modulus. It is important to note that a “fairly low” thermal coefficient of elastic modulus is not sufficiently “low” enough to support the resolution and accuracy demands that are the object of this invention.
U.S. Pat. No. 6,080,939 teaches a similar method of material selection and or treatment for a metallic oscillating element construction. It should be noted that the methods described in U.S. Pat. No. 6,080,939 are well known in the art of the manufacture of bourdon tube pressure gages and precision mechanical resonating structures, precision springs, tuning forks, vibration based pressure transducers, vibrating densitometers, and other precision elastic components.
Nearly every high quality mechanical bourdon tube type pressure gage, Heise, Aschcroft, Rosemount, and others utilize materials and procedures very similar to those discussed in U.S. Pat. No. 4,836,314.
U.S. Pat. Nos. 3,946,615 and 4,048,846 detail methods of addressing the thermal coefficient of elastic modulus with specific heat treatment methods with materials similar to Ni-Span-C. The now public manufacturing methods, from 1959, for the Bulova “Accutron” watch, that utilized a mechanical tuning fork, required materials and procedures similar to those discussed in U.S. Pat. Nos. 3,946,615 and 4,048,846 as well.
A vibrating level detection system for the indication of pressure including the compensation of the thermal coefficient of the elastic modulus are described in U.S. Pat. No. 4,311,053. It should be noted that this device actually takes advantage of the change in the resonant frequency of the system with pressure.
U.S. Pat. No. 6,502,450 states in part that “[t]o compensate for instrument effects in direct mass measurements, a differential particulate mass measurement microbalance employing a pair of oscillating quartz crystal detectors has previously been proposed.”
In this previously proposed approach, a particle laden gas stream impacts upon the first detector and a particle free gas stream impacts the second detector. The second mass detector is used as a reference to cancel out detector instrument effects from a mass reading provided by the first detector. However, the first and second detectors are not of a geometry capable of addressing the additional problem of the density change of the fluid within the hollow oscillating element and therefore can not compensate for these effects. Additionally, the idea of impacting a clean and particulate laden gas stream requires removing particulate from one stream and leaving the other stream intact. This scheme requires a second particulate collection means. The subject invention, however, does NOT require a second particulate collection means.
U.S. Pat. No. 5,571,945 discloses a similar measurement approach employing a pressure sensor to measure a pressure differential between a pair of particulate matter collectors. This patent also requires removing particulate from one stream and leaving the other stream intact. Additionally, this scheme requires a second collection means.
Neither of these two aforementioned patents adequately addresses the adverse effects of indicating a mass because both subject the “reference” resonator to a change in state resulting from the collection of a “clean” sample.
U.S. Pat. No. 5,349,844 discloses a similar approach for use with a filter that is caused to oscillate in a direction substantially perpendicular to a plane of the filter.